In the field of composite structures, in particularly those structures which are made up of layers of tape plys, it is essential to be able to precisely cut the material at predetermined times, to minimize scrap and to insure control of the remaining tape fabric on a roll assembly.
Prior art assemblies have utilized tape composite fabric which is carried on a paper backing, and the paper and tape are slid across a platen which opposes a knife assembly. The knife assembly has a presser foot which rides on the surface of the tape to be cut, and the knife point extends from the presser foot an amount equal to the thickness of the tape. By this type of assembly, it is expected that the tape will be of uniform thickness, and the presser foot and knife assembly will cause the tape to be cut, while leaving the backing paper intact. However, certain difficulties are encountered in actually applying this type of assembly. First, the tape is generally of varying thickness due to manufacturing tolerances, so that the presser foot which rides on the surface of the tape, will cause the knife point to either penetrate entirely through the paper backing, (or at least far enough to weaken the paper backing to the point where it breaks), or a thicker amount of tape will cause the presser foot to ride too high and thus the high knife point will not fully sever the tape.
The paper backing is generally held to a much closer manufacturing tolerance for its thickness, because of the mature development of the paper tape art. In contrast, contemporary composite materials are generally difficult to form to a very precise thickness; and, it is not generally necessary for the composite tape to be held to precise thicknesses, since the final assemblies of tape layers are generally built up and compacted in a press. The materials which make up the tape fabric may typically comprise carbon fibers and the like, impregnated with a prepreg material such as epoxy. This type of tape fabric is somewhat difficult to handle because it is in a tacky state before final curing in a press. For this reason, the material is additionally difficult to maneuver a knife assembly against, since a presser foot must ride smoothly on the tacky surface.
It may be appreciated that other types of tape may be employed, such as reinforced polyetheretherketone (PEEK), which is a thermoplastic material.
Applicant has obviated the difficulties inherent in the prior art knife assemblies for tape laying machines, by a novel design of presser foot and knife which are relatively moved with respect to one another while both are traversing a composite tape.